[SOLVED] What is impedance? Why do PCB design need impedance matching?

[gavinray]

When designing high-speed PCBs, we all know the concept of impedance. So why do we need to control impedance in high-speed PCB design, and which signals need controlled impedance and what is the impact of uncontrolled impedance on our circuit?

 

[KlausST]

Hi,

Rather basic questions. Not easy to answer in a short forum reply.
Thus there are thousands of tutorials and even videos that explain the topic.

Generally: impedance is nothing just related to PCBs.
It also relates to cables and connectors, antennas, transmitters and receivers.
Even more than 50 years ago you had to choose the correct antenna, the correct connector, the correct cable (all related to impedance)
... to get best radio quality.

And impedance is not only related to electronics.
It also relates to acoustics. You can imagine "impedance" as how "hard" the signal is transferred.

Let's stay at acoustics:
Our ears are made for "high impedance" air.
Thus it's much harder for us to communicate in "low impedance" water.
Dolphins are used to water, they will have problems in the air.
If there is an impedance "mismatch" this does not mean that transmission is impossible at all, it's just less effective.
So humans can speak and hear under water .... and you can hear dolphins in air.

There also is a relationship between "impedance" and speed of signal.
In air you get about 340m/s, in water 1440m/s, iron 5100m/s.

On every impedance mismatch (different medium) only a part if the energy is transmitted into the new medium, the other oart is reflected as echo.
(Electric as well as acoustic)
The more echo you have the harder it is to understand the information (electric as well as acoustic)

So: on every impedance mismatch you get
* less energy to be transferred (forward)
* an echo (back)
* again from every existing echo a part is forwarded and a part is reflected as new echo
--> the ratio of "signal to echo" becomes worse

Mind: the start (transmitter) may have a mismatch, the end, every branch, and every end of branch...

As already mentioned: watch a couple of videos.

Klaus